For an internal-combustion engine mounted on a vehicle, since a throttle valve arranged to cross an air-intake pipe is configured to be closed typically when slowing down the vehicle, the air-intake pipe is substantially blocked. Then, a space inside the air-intake pipe downstream of the throttle valve drops to a negative pressure because the engine continues running (i.e., also continues emitting exhaust gas). This results in carrying out combustion under a condition in which oxygen runs short, and as a result misfire or excessive fuel supply may occur. Misfire and excessive fuel supply are not desirable because they cause an increase in HC or CO in the exhaust gas; therefore, they cause an increase in temperature of a catalyst to deteriorate the catalyst.
Japanese Unexamined Patent Application Nos. HEI 05-240095 and HEI 09-4500 disclose methods of thinning combustion during a low load operation (especially, during idling).
Japanese Unexamined Patent Application Nos. HEI 05-240095 and HEI 09-4500 describe a control for thinning combustion only when an idling state of the engine is detected; however, no measure to detect vehicle deceleration and base thinning of combustion on a detected vehicle deceleration is described.
Although misfire or excessive fuel supply may occur during deceleration of the vehicle similar to the case of the idling state, when the throttle valve is opened from such a condition and is moved to an acceleration condition, the misfire or excessive fuel supply may be easily repeated since the temperature inside the combustion chamber is dropped because of the previous misfire or excessive fuel supply. In due course, when the temperature inside the combustion chamber increases to a sufficient temperature for combustion, a sudden combustion takes place and, therefore it causes an acceleration shock or a torque variation since a lot of oxygen is supplied into the combustion chamber.
On the other hand, when accelerating from the idling state, the combustion chamber can maintain its temperature high enough for combustion. In addition, a clutch connection exists. Thus, the acceleration shock or torque variation does not cause an adverse effect.